Diffuser for turbomachines

ABSTRACT

A diffuser is described with an entry section close to the last stage of a turbine having an outer wall including several subsequent straight wall parts at an angle to each other followed by a curved wall part with the curved wall part connecting the straight wall parts to a diffuser lip or exit and having as vertical cross-section a non-circular 2 nd  order curve, preferably part of an elliptic curve.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application SerialNo. 12172393.6; filed on Jun. 18, 2012, entitled “DIFFUSER FORTURBOMACHINES” which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to improvements to a diffuser for aturbomachine. It is particularly, but not exclusively, relevant to thediffuser following the last stage of a low pressure (LP) steam turbine.

BACKGROUND OF THE INVENTION

Diffusers as found in large steam turbines are used for example to guidethe steam from the last stage of the turbine to a condenser. Such adiffuser has two or more essentially concentric walls arranged at leastinitially with an axial orientation around the rotor axis of theturbine.

As described for example in the U.S. Pat. No. 6,602,046 the diffuserfollowing the last stage has tasks of decelerating the flowing medium,increasing the usable pressure or enthalpy drop across the turbines,converting a proportion of the kinetic energy to pressure energy andreducing flow losses at the diffuser outlet toward the condenser groups.It is hence clear that the design of the diffuser contributes to theoverall efficiency of a turbine machine and for that reason many effortshave been made to optimize the diffuser layout.

In a document published at the GT2011, Proceedings of ASME Turbo Expo2011, Jun. 6-10 2011 in Vancouver BC, Ca. by Ch. Musch, H, Stuer and G.Hermle entitled “OPTIMIZATION STRATEGY FOR A COUPLED DESIGN OF THE LASTSTAGE AND THE SUCCESSIVE DIFFUSER IN A LOW PRESSURE ENVIRONMENT, theauthors present a diffuser design with outer wall cross-sectionconsisting of two straight lines at an angle followed by an arc andanother straight section at an outlet in radial directions.

As described in the '046 patent, the straight lines can be formed as asequence of kinks in the wall of the diffuser to deliberately cause flowseparation from the wall. The configuration allows shock boundary layerpulsations to be suppressed. However, the measure may be associated witha considerable reduction in the diffuser efficiency.

In view of the existing prior art it can be seen as an object of thepresent to further optimize the existing diffuser designs and thusincrease the efficiency of the turbomachines, particularly low pressuremodules or turbines of a steam driven power plant.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided adiffuser having an outer wall having an entry section close to the laststage of a turbine including several subsequent straight sections at anangle to each other followed by a curved section with the curved sectionhaving as vertical cross-section a 2^(nd) order curve, preferable asegment of an ellipse but excluding circle segments.

A 2^(nd) order curve excluding circles can be described for example asan algebraic equation of Cartesian coordinates in the form:

a ₁₁ x ² +a ₁₂ xy+a ₂₂ y ²+a ₁₃ x+2a ₂₃ y+a ₃₃=0   [1]

with at least one mixed coefficient being <>0.

It is found that using an elliptical shaped diffuser lip, a betterdiffuser design in terms of aerodynamics and performance is madepossible. Particularly it is possible to have a higher curvature at thein-flow direction at the first part of the curved section and a reducedcurvature towards the diffuser outlet. It is further possible tooptimize the flare (or opening) angle of the diffuser and area ratiomuch closer to an ideal value than with an arc as due to the tip jet ofthe last stage blade, more turning of the flow in the diffuser can bedone within the first angled straight sections or kinks.

These and further aspects of the invention will be apparent from thefollowing detailed description and drawings as listed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 shows a schematic vertical cross-section of a diffuser for a lowpressure steam turbine stage as known with potential optimizationparameters;

FIG. 2 shows a schematic vertical cross-section of a diffuser inaccordance with an example of the invention overlaid over the knowndiffuser of FIG. 1; and

FIG. 3 is a plot of slice diffuser recovery illustrating the potentialefficiency gains of diffusers in accordance with the present inventionover known diffusers.

DETAILED DESCRIPTION OF THE INVENTION

Aspects and details of examples of the present invention are describedin further details in the following description using the example of adiffuser for the a low pressure steam turbine.

In FIG. 1 there is shown a schematic vertical cross-section of adiffuser for a low pressure steam turbine stage as proposed in theGT2011 publication as referenced above. The figure shows a cross-sectionof the upper half of an essentially rotationally symmetrical diffuser.It also shows a part of the rotor 10 and of the inner casing 11. Betweenthe rotor and the casing and attached to one, respectively, are rotatingblades and stator blades. The last stage of the turbine includes acircumferential arrangement of stator blades 111 attached to the casing11 and a circumferential arrangement of rotating blades 101 attached tothe rotor 10.

Following the last stage 111,101 is the diffuser 12 . It has an outerwall 121 forming an inner radius of the diffuser and an inner wall 122,which together form an annular conduit guiding the steam to a condenser(not shown) and each extend to a diffusor lip 124 that together forms adiffusor exit 123. The enlarged detail of FIG. 1 shows a part of theouter wall 121 of the diffuser and illustrates the parameters which canbe used to optimize the diffuser for a given turbine and steam flow.Following the direction of the steam flow, the wall has first straightpart of length I1 followed by a second straight part of length I2. Thesecond straight part forms an angle 81 with respect to a horizontalline, i.e. in axial direction. The two straight parts are followed by acurved part. The curved part has an angle δ2 with respect to ahorizontal line at its entry and a radius of R and an angle δ3 withrespect to a horizontal line at its exit. The arc is followed by anotheressentially straight section with the length I3 in direction of the exit123. As described in the GT2011 reference, all the parameters shown canbe altered in order to configure an optimized diffuser 12.

In FIG. 2 a modified diffuser in accordance with an example of theinvention is shown overlaid over the diffuser of FIG. 1 (shown as dashedlines 121). The new diffuser has a higher number of straight parts (3over 4) and a higher order curve 21 following the straight parts. Thecurve 21 is essentially an elliptical curve.

As shown in FIG. 2, the elliptical shape 21 has a higher curvature atthe beginning or entry and flares out into a flatter part towards theexit 123 compared to the circular curve 121 (dashed). Other numerals inFIG. 2 are the same as in FIG. 1. when denoting identical or similarelements.

A comparison of the efficiency of the two designs of FIG. 2 is shown inFIG. 3 illustrating the slice diffuser recovery. The graph is a plot ofthe recovery Chi between the diffuser inlet (last stage blade exit) andthe diffuser exit (2R plane) over the medium speed in axial directionC_(zω)in m/s at the diffuser inlet. The upper line illustrates therecovery of the new diffuser. This recovery is at least 3 percent abovethe line of a known diffuser as shown in FIG. 1. From this comparison itbecomes clear that hybrid diffusers in accordance with the inventionhave the potential to improve diffuser recovery in a significant manner.The hybrid diffuser with 4 or more straight parts followed by anelliptical can be regarded as the one with a better improvementpotential than the prior art diffuser of FIG. 1.

Similar improvements can be demonstrated when comparing the new diffuserwith a design using multiple straight parts, e.g. seven or more, for theouter wall while omitting any curved part.

A diffuser with a higher number of straight parts and their respectivelengths angles between them followed by a curve of the general shape

a ₁₁ x ² +a ₁₂ xy+a ₂₂ y ²2a ₁₃ x+2a ₂₃ y+a ₃₃<0   [1]

with at least one mixed coefficient not being zero has a higherpotential of being further improved than the more limited designsfollowing the prior art. Such optimisation can be made using any of theknown tools such as ANSYS CFX or other methods as described in the abovereferenced GT2011 publication.

The present invention has been described above purely by way of example,and modifications can be made within the scope of the invention. Theinvention may also comprise any individual features described orimplicit herein or shown or implicit in the drawings or any combinationof any such features or any generalization of any such features orcombination, which extends to equivalents thereof. Thus, the breadth andscope of the present invention should not be limited by any of theabove-described exemplary embodiments.

Each feature disclosed in the specification, including the drawings, maybe replaced by alternative features serving the same, equivalent orsimilar purposes, unless expressly stated otherwise.

Unless explicitly stated herein, any discussion of the prior artthroughout the specification is not an admission that such prior art iswidely known or forms part of the common general knowledge in the field.

1. A diffuser following the last stage of a turbine; said diffuser comprising: a diffuser lip; a curved wall; and an outer wall that extends to the diffusor lip to form a diffusor exit, the outer wall including several subsequently connected straight wall parts at an angle to each other followed by a curved wall part, wherein a curved section is located between the straight wall parts and the diffuser lip and has a vertical cross-section that forms a second order curve.
 2. The diffuser of claim 1, wherein the second order curve is not a circle segment.
 3. The diffuser of claim 1, wherein the second order curve is a segment of an elliptical curve.
 4. The diffuser of claim 1, wherein the number of straight parts is four or more.
 5. The diffuser of claim 1, wherein the second order curve is replaced by a larger number of straight parts. 